The present invention relates to a light-emitting element, such as a light-emitting diode and a semiconductor laser element, that emits light in a bluish purple range or an ultraviolet range, or a semiconductor device such as a transistor that is operated in the order of the GHz, and also concerns a fabricating method for such a device.
A nitride semiconductor, which is indicated by a general formula, BxAlyGa1-y-zIn2N (0=x=1, 0=y=1, 0=z=1), is a semiconductor that exerts a great wide-bandgap energy, that is, a bandgap energy of 3.4 eV (at room temperature) in the case of GaN; therefore, this semiconductor is expected as a material that can realize visible-range light emission in a wide area ranging from blue to ultraviolet. Moreover, since this semiconductor exerts a large electron velocity in the high electrical field, it is also expected as a material for a high-temperature operative and high-output transistor.
Conventionally, since the nitride semiconductor generally has a high growth temperature and since no substrate material that lattice-matches therewith is available, a nitride semiconductor having a desirable crystal was not obtained. However, since a technique in which a nitride semiconductor is grown on a sapphire substrate through a low-temperature buffer layer by using an MOCVD method has been developed, its crystallinity is improved so that light-emitting diodes and semiconductor lasers using this material have been commercialized. In general, in the GaN crystal formed on a sapphire substrate, there are crystal defects having a size of about 1×109 cm−2; however, in a blue-light emitting device using InGaN as a light-emitting layer, since carriers are located biasly due to unevenness in the In composition, blue-light emission is available even when its crystal defect density is high.
However, since the crystal defect acts as a non-radiative recombination center of carriers, the light-emitting efficiency is lowered to cause degradation in the reliability of the light-emitting element. In order to solve this problem, a crystal-defect reducing technique utilizing growth in the lateral direction (Epitaxial Lateral Overgrowth) has been developed. For example, GaN is allowed to laterally grow on a mask such as a SiO2 film or after a step difference has been formed on a substrate, GaN is allowed to laterally grow on the step difference so that the crystal defect density can be reduced to about 1×107 cm−2.
In this manner, by reducing the crystal defect density of a nitride semiconductor, the nitride semiconductor device is greatly improved in its characteristics so that various researches and developments for reduction in crystal defects have been actively carried out.
Moreover, in addition to the crystal-defect reducing technique, another technique, which has drawn public attention as a performance-improving method for nitride semiconductor devices, is a selective oxidizing technique for the nitride semiconductor surface. For example, an oxide layer is formed on a GaN surface by carrying out a thermal process in an oxygen atmosphere using an Si thin film or the like as a mask material, and the mask material is then removed. When a field-effect transistor is further formed on the GaN surface, the oxide layer formed on the GaN surface allows element separation of the device and provides high pressure resistance to the device (for example, see Japanese Patent Application Laid-Open No. 2001-26755). This selective oxidizing technique can be applied to current constriction of a semiconductor laser, and is expected to be used in a wide range of applications.
Here, in the ultraviolet light-emitting device, AlGaN, which is transparent to ultraviolet rays, needs to be used as a base member. In this case, when GaN is allowed to grow laterally by using a SiO2 film as a mask, polycrystal is deposited on the SiO2 film, with the result that GaN is not selectively grown, making it difficult to reduce crystal defects.
Moreover, since a mixed crystal of AlGaN without containing In is used as an activator layer of an ultraviolet light-emitting device, localization of carriers occurs only in a small level. For this reason, a reactive current flowing through the crystal defect appears conspicuously, resulting in a low light-emitting efficiency.
Furthermore, in general, the manufacturing processes of the device include processes in which a step difference is formed by using dry etching so that electrodes are formed on the step difference in most cases. Consequently, on an interface formed through these processes, the reactive current flowing through etching damages increases, causing degradation in the element characteristics.